想在centos下开放端口结果在启动防火墙时出现问题了
启动防火墙时遇到的问题如下：

root@ecs-d5cf-0015 ~]# systemctl start firewall.service
Failed to start firewall.service: Unit not found. 

是因为没有安装firewalld
依次输入下列命令
（使用yum install firewalld命令安装时，中间停顿输入y既可）

yum install firewalld
systemctl unmask firewalld
systemctl enable firewalld
systemctl start firewalld

systemctl start firewalld.service //开启防火墙
systemctl stop firewalld.service //关闭防火墙
systemctl enable firewalld.service //设置开机自动启动
systemctl disable firewalld.service //设置关闭开机制动启动
firewall-cmd --reload //在不改变状态的条件下重新加载防火墙